1. Field of The Invention
This invention relates to the stabilization of wet process phosphoric acid with respect to the post-precipitation of solids during storage and shipment of the acid.
2. Prior Art
Most phosphoric acid produced in the United States is produced by the wet process. In this process finely ground phosphate rock is slurried with sulfuric acid and recylced dilute phosphoric acid. The products produced by the chemical reaction between the phosphate rock and sulfuric acid are orthophosphoric acid (commonly known in the industry as phosphoric acid, or, more particularly, as wet process phosphoric acid), gypsum, and numerous other suspended and dissolved impurities. The slurry produced is filtered to remove solid impurities, of which gypsum is the major constituent. The resulting filtrate usually contains between about 25 and 35% by weight P.sub.2 O.sub.5, and between about 1 and 8% by weight suspended solids. This dilute wet process acid is usually concentrated in multiple stage evaporators to about 52-54% P.sub.2 O.sub.5 prior to storage and eventual shipment. During processing, storage and shipment the impurities contained in the wet process acid settle out to form a hard, voluminious sludge. This sludge formation is known in the industry as post-precipitation. The sludge contains a high proportion of iron and aluminum phosphate compounds. One of the major constituents of the sludge is the complex salt (Al,Fe).sub.3 KH.sub.14 (PO.sub.4).sub.8.4H.sub.2 O. The sludge is difficult and costly to remove and, additionally, represents substantial losses of P.sub.2 O.sub.5 values.
The impure state of wet process phosphoric acid is well known, and the post-precipitation of solids is a recognized problem. Many shipping grade acids are partially clarified by settling to reduce the sludge content prior to shipment. Settling, however, does not resolve the problem easily or economically because of the lengthy time it requires. For example, one month unassisted settling of 100 tons P.sub.2 O.sub.5 per day as 54% P.sub.2 O.sub.5 acid requires a minimum storage capacity of 750,000 gallons. Solids can be removed in shorter periods of time by centrifugation, but this still does not solve the problem of post-precipitation because acids clarified solely by centrifugation still exhibit excessive post-precipitation tendencies.
Minimizing post-precipitation may also be accomplished by various means of acid purification such as chemical precipitation, solvent extraction and ion exchange; however, these processes are complex and expensive.
One solution to the post-precipitation problem has been to combine clarification with other process operations so as to produce an acid in which post-precipitation is substantially inhibited. Such an acid is known in the industry as a stabilized acid. Considerable research has gone into developing processes for the manufacture of wet process phosphoric acid stabilized with respect to the post-precipitation of solids. One well known stabilization process is the Hill process, described in U.S. Pat. Nos. 4,110,422, 4,164,550, 4,248,846, 4,279,877, 4,293,311, 4,305,915 and 4,364,912.
U.S. Pat. No. 4,110,422 describes the basic Hill process, in which a stabilized wet process phosphoric acid is produced by addition of an aluminum silicate material such as perlite to clarified dilute phosphoric acid of about 22-35% P.sub.2 O.sub.5 strength, concentrating the acid to about 42-52% P.sub.2 O.sub.5, transfering it to a crystallization zone where additional clarification occurs, and then concentrating it to about 63% P.sub.2 O.sub.5 strength.
U.S. Pat. No. 4,164,550 covers the same process as U.S. Pat. No. 4,110,422, but the P.sub.2 O.sub.5 strength range of the feed acid to the process is 36-46%. U.S. Pat. No. 4,248,846 improves on the basic Hill process by the incorporation of a recycle stream from the crystallizer underflow to the acid train, addition of sulfuric acid to evaporators when processing rock high in iron and aluminum, and cooling one or more streams of the process. U.S. Pat. No. 4,279,877 modifies the Hill process for conditions of high-iron feed acid in which some of the iron is present in the ferrous form. If the Fe.sub.2 O.sub.3 -to-P.sub.2 O.sub.5 weight ratio is higher than 0.04, an oxidant such as hydrogen peroxide is used to oxidize all ferrous iron to the ferric state. The treatment reduces post-precipitation of the final product acid. U.S. Pat. No. 4,293,311 also modifies the basic Hill process by incorporation of a crystallizer underflow recycle stream to the aluminum silicate addition vessel. This patent specifies that no oxidant is necessary if the Fe.sub.2 O.sub.3 -to-P.sub.2 O.sub.5 weight ratio is between 0.03 and 0.04. Aluminum silicate is still required, however. U.S. Pat. No. 4,305,915 covers the composition of matter made by the above patents and having desirable characteristics with respect to its post-precipitation tendencies. The composition of matter is made by a process that requires the use of perlite or similar aluminum silicate.
U.S. Pat. No. 4,364,912 discloses the production of a stabilized acid without the use of aluminum silicate by prescribing a Fe.sub.2 O.sub.3 -to-P.sub.2 O.sub.5 weight ratio lower than 0.03 in the feed acid with all other steps of the basic Hill process unchanged.
All of the processes described above require an initial clarification step. Most require the addition of aluminum silicate, and some require addition of an oxidant. Both the initial clarification step and the addition of aluminum silicate and oxidants add significantly to the cost of producing a stabilized acid. The addition of these steps also interferes with the simultaneous production of unstabilized grades of phosphoric acid such as normal merchant grade wet process acid and clarified merchant wet process phosphoric acid. It is apparent that a stabilization process not requiring these steps would be economically desirable. It is an object of this invention to provide one such process.
It is also an object of this invention to provide a wet process phosphoric acid substantially free of post-precipitation.
It is also an object of this invention to provide a wet process phosphoric acid that will remain free of post-precipitation for a long period of time.
Another object of this invention is to provide a process for stabilizing wet process phosphoric acid which process produces an excellent quality acid with respect to post-precipitation characteristics from high-iron acid feeds as well as from low iron acid feeds without aluminum silicate addition and without oxidation or any other such special treatment.
Another object of this invention is to provide an inexpensive process for producing wet process phosphoric acid stabilized with respect to post-precipitation.
Still another object of this invention is to provide a wet process phosphoric acid with higher-than-normal P.sub.2 O.sub.5 content in order that superior liquid fertilizers can be produced from the acid.
A still further object of this invention is to provide a process for stabilizing wet process phosphoric acid which process will not interfere with the simultaneous production of unstabilized normal merchant grade wet process phosphoric acid and unstabilized, merchant clarified wet process phosphoric acid.
These and other objects are accomplished by the process of our invention and will become apparent to those skilled in the art from the description that follows.